It is known, from U.S. Pat. No. 3,138,560, that if the palladium is adsorbed from a solution of sodium tetrachloropalladate or palladium chloride, most of the palladium is reduced immediately and deposited on the surface in the form of a thin film of palladium metal.
The resulting catalysts are scarcely active. Reduction of palladium to metallic palladium is attributed to functional groups, such as the aldehyde groups that are present on the surface of the activated carbon.
In order to avoid this reduction, palladium compound solutions have been used which contain an oxidizing agent, such as hydrogen peroxide.
In order to be able to deposit on the surface of the activated carbon palladium crystallites having a relatively small size (smaller than 35 .ANG. if possible), solutions of the palladium compound in organic solvents have been used (U.S. Pat. No. 4,476,242). In this case, all of the palladium is concentrated in a layer less than 70-80 microns thick.
Results similar to those mentioned above are achieved if the palladium is deposited from aqueous solutions of sodium tetranitropalladate (U.S. Pat. No. 4,421,676).
According to conventional methods, the catalyst is prepared by washing the activated carbon with water in order to eliminate the fines; the carbon thus treated is dispersed in water, optionally correcting its basicity, and the aqueous solution of the palladium salt is then added thereto drop by drop.
The impurities contained in the terephthalic acid obtained by oxidation of p-xylene are consist primarily of paratoluic acid and 4-CBA.
Whilst paratoluic acid can be removed by cooling and crystallization of the terephthalic acid solutions that contain it, removal of 4-CBA requires transformation by reduction to compounds which can be separated by crystallization.
The transformation is performed by hydrogenation on palladium catalysts on an activated carbon support.
The catalysts must have high activity and selectivity in order to reduce the concentration of 4-CBA to levels acceptable to users of terephthalic acid and to convert said 4-CBA into compounds that can be separated by crystallization.